1. Field of the Invention
The present invention relates to a transmission apparatus using asynchronous serial communication, a reception apparatus using asynchronous serial communication, a serial communication apparatus using asynchronous serial communication, and a printing apparatus including the serial communication apparatus.
2. Description of the Related Art
Conventionally, in serial communication which adopts an asynchronous communication method, there is a frequency deviation between a transmission clock on the transmission side and a reception clock on the reception side. Using, for example, a method described in Japanese Patent Laid-Open No. 2007-336190, therefore, reception of data is made more reliable, and synchronization correction for communication is performed.
In the conventional example described above, however, reception data is sampled with a sampling clock with a frequency higher than that of a communication clock, and it is determined based on the sampling result for one bit that data including a larger number of components “0” or “1” is determined as reception data. In principle, therefore, an error due to a frequency deviation is accumulated in subsequent bits. That is, a sampling result includes more errors towards the subsequent bits.
Assume that a communication unit (for example, a unit of a start bit/data/parity bit/stop bit) is defined as one frame. In this case, when an error with a ½ bit or more is accumulated while one frame is received, the frame cannot be correctly received.
FIG. 15 is a view showing the structure of one frame in asynchronous serial communication.
Since one frame is defined by a start bit (one bit), data (eight bits), a parity bit (one bit), and a stop bit (one bit) as shown in FIG. 15, it can only be correctly received up to a deviation of ±5%. Furthermore, if the number of bits of data in one frame increases, a tolerable deviation becomes smaller, as a matter of course.
As countermeasures against EMI (electromagnetic noise), there is conventionally proposed that an attempt is made to reduce radiation noise generated by a transmission signal by fluctuating a transmission clock using an SSCG (Spectrum Spread Clock Generator) technique. Even in this case, however, it is impossible to achieve sufficient noise reduction since a clock cannot fluctuate so much because of the tolerable deviation.